This invention concerns methods for making investment casting molds comprising imaging agents in at least the facecoat of the mold, and methods for imaging inclusions in metal or metal alloy articles made using such molds.
Investment casting is a process for forming metal or metal alloy articles (also referred to as castings) by solidifying molten metal or alloys in molds having an internal cavity in the shape of such articles. The molds are formed by serially applying layers of mold-forming materials to wax patterns formed in the shape of the desired article. The first layer applied to the pattern, referred to as the facecoat, contacts the metal or metal alloy being cast during the casting process. Materials used to form the facecoat, and perhaps other xe2x80x9cbackupxe2x80x9d layers of the mold, can flake off the mold and become embedded in the molten metal or alloy during the casting process. As a result, the metal or alloy article includes a material or materials not intended to be part of the article, such material or materials being referred to as xe2x80x9cinclusionsxe2x80x9d.
Many industries, particularly the aerospace industry, have stringent specifications as to the acceptable content and/or size of inclusions. The location of inclusions in castings can be difficult, and in some cases prior to the present invention, impossible to detect. Some inclusions, if detected, can be removed from the metal article, and the article repaired, without compromising its structural integrity.
Titanium has been used by the investment casting industry primarily for casting articles having relatively small cross sections. However, investment casting is now being considered for producing structural components of aircrafts having significantly larger cross sections than articles cast previously. Certain inclusions in relatively thin articles can be detected using X-ray analysis. For example, thorium oxide and tungsten have been used as refractories to produce mold facecoats for investment casting. Some thorium oxide and tungsten inclusions could be detected in titanium castings by X-ray analysis because there is a sufficient difference between the density of thorium oxide and tungsten and that of titanium to allow imaging of thorium-oxide or tungsten-derived inclusions. This also generally has proved true of articles having relatively small cross sections cast using molds having yttria facecoats. The difference between the density of yttria and that of titanium is sufficient to allow detection in relatively thin parts, such as engine components. But, X-ray detection cannot be used to image yttria inclusions in titanium or titanium alloy articles as the thickness of articles produced by investment casting increases beyond some threshold thickness that is determined by various factors, primarily the thickness of the cast part, the type of metal or alloy being cast, the size of the inclusion and the material or materials used to form the mold. Inclusions also cannot be detected by X-ray if the difference between the density of the facecoat material and the metal being cast is insufficient or if the size of the inclusion is very small.
Thermal neutron radiography (N-ray) imaging agents have been used in the casting industry prior to the present invention. For example, ASTM (American Society for Testing and Materials) publication No. E 748-95 states that xe2x80x9c[c]ontrast agents can help show materials such as ceramic residues in investment-cast turbine blades.xe2x80x9d ASTM E 748-95, p.5, beginning at about line 46. This quote refers to the detection of ceramic residues by N-ray on articles having an internal cavity produced by initially solidifying metal about a ceramic core. The ceramic core is removed to form the cavity, and thereafter a solution of gadolinium nitrate is placed in the cavity. The gadolinium nitrate solution remains in the cavity long enough to infiltrate porous ceramic core residues that are on the surface of the article. The residues then can be imaged by N-ray. However, this method does not work for imaging inclusions.
The present invention addresses the problem of imaging inclusions embedded in relatively thick castings. One feature of the method is the incorporation of an imaging agent into the investment casting mold, particularly in the facecoat of the mold, prior to casting so that inclusions can be imaged in the cast article.
One embodiment of the present method first involves providing a cast metal or metal alloy article made using a casting mold comprising an imaging agent in amounts sufficient for imaging inclusions, and thereafter determining whether the article has inclusions by N-ray analysis. The step of providing a cast metal or metal alloy article may comprise providing a casting mold comprising an N-ray imaging agent, and then casting a metal or metal alloy article using the casting mold. Typically, the mold facecoat, and perhaps one or more of the mold backup layers, comprises an imaging agent distributed substantially uniformly throughout in amounts sufficient for imaging inclusions. The article is then analyzed for inclusions by N-ray imaging. The method also can include the step of analyzing the metal or metal alloy by X-ray imaging. The method is particularly suitable for detecting inclusions in relatively thick articles, such as titanium or titanium alloy articles, where at least a portion of the article has a thickness of greater than about 2 inches, particularly facecoat inclusions in titanium castings. An xe2x80x9cinclusionxe2x80x9d can refer to materials not desired in the casting, such as inclusions derived from the mold facecoat. Alternatively, an xe2x80x9cinclusionxe2x80x9d can also refer to materials that should be included in the casting, such as strength-enhancing fibers, in which case the fibers can be coated with imaging agent, or intimate mixtures of fibers and imaging agents can be made and used. Detected deleterious inclusions are removed by conventional means.
Simple binary mixtures comprising an imaging agent or agents and a mold-forming material or materials can be used. The present method preferably involves forming an intimate mixture of the materials used to practice the present invention, such as intimate mixtures of refractory materials, intimate mixtures of imaging agents, and/or intimate mixtures of imaging agent or agents and a refractory or refractory materials. Intimate mixtures can be produced in a number of ways, but currently preferred methods are to either calcine or fuse the mold-forming material, such as yttria, with the imaging agent, such as gadolinia.
Alternatively, solutions of imaging agents can be used to infiltrate the mold (as opposed to a casting) prior to casting the metal article. For example, solutions comprising nitrate, halide, sulfate, perchlorate salts of imaging agents can be used to form solutions comprising such materials, and these solutions are then used to infiltrate an investment casting mold. The infiltration process can be enhanced by placing the mold in a chamber which can be evacuated, at least partially. This facilitates having imaging agent solution enter the pores of the mold.
The difference between the linear attenuation coefficient of the article and the linear attenuation coefficient of the imaging agent should be sufficient to allow N-ray imaging of the inclusion throughout the article. The imaging agent typically includes a material, usually a metal, selected from the group consisting of lithium, boron (e.g., TiB2), neodymium, samarium, europium, gadolinium, dysprosium, holmium, erbium, ytterbium, lutetium, iridium, boron, physical mixtures thereof and chemical mixtures thereof. Examples of suitable imaging agents comprising such metals include metal oxides, metal salts, intermetallics, and borides. Gadolinia is a currently preferred imaging agent for imaging inclusions in titanium or titanium alloy castings.
The refractory material used to make the facecoat slurry typically comprises from about 0.5 to about 100 weight percent imaging agent, more typically from about 1 to about 100 weight percent, even more typically from about 1 to about 65 weight percent, and preferably from about 2 to about 25 weight percent, imaging agent.